1. Field of the Invention
The present invention relates to a gallium nitride based compound semiconductor light emitting element capable of emitting light beams in the blue to ultraviolet ranges, and more specifically to an electrode structure thereof.
2. Description of the Background Art
A schematic cross sectional view of FIG. 5 shows a stacked structure of an example of a conventional gallium nitride based compound semiconductor light emitting element (see Japanese Patent Laying-Open No. 9-129919). In this light emitting element, on an insulating sapphire substrate 100, an n type contact layer 200, a light emitting layer 300, a p type clad layer 400 and a p type contact layer 410 are stacked in this order. On a main region on a top surface of p type contact layer 410, a first p type electrode layer 500 of metal is formed. On a first region on the top surface of the first p type electrode layer 500, a second p type electrode layer 600 of a transparent conductive film containing oxide is formed, and on the remaining second region, a pad electrode 700 is formed. Further, a portion of n type contact layer 200 is exposed by anisotropic etching, and an n type electrode 800 is formed on the exposed region.
In the electrode structure of the light emitting element such as shown in FIG. 5, the top surface of the first p type electrode 500 is covered either by the second p type electrode layer 600 or the pad electrode 700. Side surfaces of the first p type electrode 500, however, are exposed to the atmosphere. Further, portions of the top surface of p type contact layer 410 that are not covered by the first p type electrode 500 are exposed.
The inventors of the present invention performed a conduction test of the light emitting element having such an electrode structure in the atmosphere and in a lump state formed of a mold resin, and it was found that when the first p type electrode layer 500 contains palladium (Pd), local bumps generate at the upper surface of p type contact layer 410 on the side surfaces and near the side surfaces of the first p type electrode layer, resulting in a particular phenomenon that the bumps turned out to be dark spaces of the light emitting region. More specifically, such a bump portion is a cause of unevenness of the light emitting pattern from the light emitting region, deteriorating reliability of the light emitting element. Further, it was found that when p type and n type pad electrodes are arranged at diagonal directions of the light emitting element, such bumps are more frequently generated on the side surfaces of the first p type electrode layer and the top surface regions of the p type contact layer 410 near the side surfaces, nearer to the region having high current density along the shortest distance between the pad electrodes.
In view of the problems experienced in the conventional light emitting element found by the inventors, an object of the present invention is to provide a nitride based semiconductor light emitting element having high reliability capable of uniform light emission in the overall light emitting regions under a low forward voltage.
According to the present invention, the nitride based semiconductor light emitting element includes at least a gallium nitride based compound semiconductor layer of a first conductivity type and a gallium nitride based compound semiconductor layer of a second conductivity type stacked on a substrate, a Pd-containing electrode is formed on a main region on the top surface of the semiconductor layer of the second conductivity type, and the top surface and side surfaces of the Pd-containing electrode as well as the surface of the semiconductor layer of the second conductivity type in an area of at least a prescribed width W from the side surfaces are covered by a conductive shielding film to be shielded from the atmosphere or a mold resin.
The Pd-containing electrode may be formed as a transparent electrode, and the conductive shielding film may be formed by a transparent conductive film.
A pad electrode may be formed on a region on the top surface of the Pd-containing electrode and, in that case, it is preferred that the conductive shielding film additionally covers the side surfaces and the peripheral portion of the top surface of the pad electrode.
A Pd-containing electrode may be formed as an ohmic electrode, and the conductive shielding film may be formed to serve additionally as the pad electrode.
The Pd-containing electrode may be formed as a single-layer or multi-layered metal thin film.
When the conductive shielding film is formed by a transparent conductive film, the thickness thereof should preferably be 0.1 xcexcm to 30 xcexcm and the aforementioned width W should preferably be at least 5 xcexcm.
When the conductive shielding film is formed to serve additionally as the pad electrode, it should preferably contain at least gold (Au), and the thickness thereof should be 0.3 xcexcm to 1.5 xcexcm and the aforementioned width should preferably be at least 5 xcexcm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.